There is widely known a photocuring technique for irradiating an ultraviolet ray (UV light) on photocurable resin and curing the photocurable resin.
In general, the photocurable resin is a liquefied resin composition containing photopolymerizable oligomer, photopolymerizable monomer, a photopolymerization initiator, and various additives. The additives are, for example, a stabilizer, a filler, and a coloring agent (a pigment).
The photopolymerizable monomer is an organic material that causes polymerization reaction according to ultraviolet ray irradiation and forms a polymer, which is a larger molecule.
The photopolymerizable oligomer is an organic material obtained by causing the photopolymerizable monomer to react in advance to form a so-called prepolymer. Like the photopolymerizable monomer, the photopolymerizable oligomer causes the polymerization reaction according to ultraviolet ray irradiation and forms a polymer.
Because the photopolymerizable oligomer has relatively high viscosity, in the photocurable resin, in general, the photopolymerizable oligomer is diluted by the photopolymerizable monomer having low viscosity.
The photopolymerization initiator is a material (called photoinitiator as well) added to start the polymerization reaction. It is known that the photopolymerizable oligomer and the photopolymerizable monomer do not cause the polymerization reaction according to irradiation of an ultraviolet ray. Therefore, the photopolymerization initiator is contained in the photocurable resin.
The photopolymerization initiator is excited according to reception of irradiation of an ultraviolet ray, cleaves, and generates a radical. By the radical, the photopolymerizable oligomer and the photopolymerizable monomer polymerize to be converted into a polymer (see, for example, Kiyoshi Kato, “Ultraviolet Curing System”, Kabushikigaisha, Sohgoh Gijutsu Sentah (United Engineering Center, Co., Ltd.), issued on Feb. 28, 1989, p. 29 to 31).
Note that, in addition to the photopolymerization initiator, an amine-based or quinone-based photopolymerization auxiliary initiator is sometimes added to the photocurable resin according to necessity.
In general, as the photopolymerization initiator, a superoxide such as a benzoin-based, acetoacetic phonon-based, or peroxide-based superoxide is used. Therefore, it is undesirable that the photopolymerization initiator, which is the superoxide, remains in resin after curing.
In particular, in recent years, an ultraviolet LED having low illuminance compared with an electric discharge lamp has been used as a light source. To supplement illuminance insufficiency, content concentration of the photopolymerization initiator tends to be higher. Therefore, the photopolymerization initiator more easily remains.
Therefore, there is known a technique for, to enable curing of photocurable resin not including or relatively not including a photopolymerization initiator, reducing oxygen in a region close to the surface of a base material, on which the photocurable resin is deposited, and increasing an amount of UV light reaching the photocurable resin (see, for example, Japanese Patent No. 4452504).